Osmolal Gap (Serum)
Info
Meas_Osm
Na
unit mg/dL mmol/L
BUN
unit mg/dL mmol/L
Glucose
unit mg/dL mmol/L
R e s u l t s
mOsm/kg serum H20
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Osmolal Gap (Serum)

Osmolal Gap = Measured Osm - Calculated Osm = Meas Osm - [(2*Na)+(BUN/2.8)+(Glu/18)]

Normal <10 mOsm/kg serum H20

Useful to group anion gap acidoses

Elevated gap: presence of significant amounts of unmeasured osmotically active solutes, incl. ethanol, methanol, paraldehyde, ethylene glycol, etc.

When Osmolal Gap due to ethanol:

  • Blood Alcohol Level (mg/dL)= 4.3 * Osm Gap.

Reference:

Purssell RA, Pudek M, Brubacher J, et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med 2001;38(6):653-9.